Pulp bleaching and refining process



Patented Jan. 9, 1940 PULP BLEACHING AND REFINING PROCESS Harold R. Murdock,'Canton, N. (2., assignorto The Champion Paper and Fibre Company, I Canton, N. 0., a corporation of Ohio No Drawing.

Application August 24, 1937, Serial No. 160,720

. 3' 401m. (or. 92-16) This invention relats to an improved process of refining and bleaching fibrous material, and

is concerned particularly with the preparation of a refined and bleached paper pulp in a form It has been common practice to complete this liberation of individual fibers at a point following' the stepsof bleaching or whitening of the brown pulp, and just prior to the formation of thebleachedpulp into a paper web. Thisrefinas, for example, the beater, the jordan, and

by means such the refiner. In addition to,bringing about fiber liberation, this same refining treatment coincidenting treatment is usually effected 5 adapted for direct use in the manufacture of ally results in that physical processing of the 5 paper, fiber board, and like manufactured prod-' flbers which is classed as hydration, and which ucts. is well known to be a requisite to the formation In the manufacture of bleached pulp from of a web of paper. raw fibrous materials, suchas, for example; wood In the usual series of operations which has just 1. and the like, it is usually thezpip'ntice to digest been described, the step of bleaching precedes the raw ma terial with a suitablewcal in such that of final fiber liberation, and consequently quantities and for such periods-' e'ftirne and at the unbleached pulp when treated with the cus-, such elevated temperatures and pressures that tomary bleaching agents contains fiber bundles, practically complete solution of theinterfibrous slivers, or shives. These fiber bundles are not I materialis efiected. E readily and uniformly penetrated by the usual I The raw fibrous material usedinthe produc bleaching agents, and'a satisfactory evenness in tion of such chemical pulps rarely is uniform in bleaching is not obtained. If the major portion chemical and physical properties. 'As a conseof the pulp is correctly bleached, then the fiber quence, the interflbrous material in various parts bundles are insumciently bleached and appear of the raw material usually is dissolved at dlfierin the white pulp as yellowish slivers or discolorent rates. According to the desired character of ations. On the other hand, if the bleaching the pulp to be produced, thedigestion treatment treatmentis made sufficiently drastic to remove may be, and usually is, continued until even the the color from these fiber bundles, the major pormore resistant portions of the raw material are tion of the pulp will have been excessively 35 substantially and thoroughly digested-in which bleached and the resultant bleached pulp will pqscase the less resistant portions of the raw masess relatively low strength characteristics.

- terial suffer an excessive digestion with a conse- It recently has been recognized that the so quent low yield. In the increasingly common called refining treatment can be done prior to the practice of developing pulps of high strength bleaching process with the advantage that the j characteristics, however, the said treatment is fiber bundles are separated into individual fibers discontinued when all but the more resistant porbefore bleaching so that a more uniform bleachtions of the raw materials have yielded to the ing operation thereby is made possible. This solvent action of the digestive agent. Particuprocess was disclosedinI-Ielder's U. S. Patent No. larly in the latter case, the resulting pulp con- 1,890,179, wherein the step of mechanical refinfii tains bundles of fibers which do not disintegrate ing is interposed between the digestion step and into separate fibers under the subsequent mild the first step involving bleaching or refining by mechanical treatment commonly. given pulps prechemical means. While this process makes pos;

liminary to bleaching; Before chemical wood sible some improvement over the prior art propulps can be converted into high quality paper cess, it does not fully realize the benefits which 40 or the like, however, it is absolutely necessary are possible by the applicants process. I

that each individual fiber be liberated from other The applicants process represents a further fibers adjoining it in the cellular structure. departure from, and improvement over, prior art Therefore in order to make unusually strong processes. It makes recognition of two points -white pulps it is not only essential that the di-. 'familiar to those well versed in the art, and more I gestion on the raw side but that all the fibers particularly so since the development and perbe liberated. This seems tube a paradoxical 'fection in recent'years of chlorination-steps in.

condition but nevertheless must be realized. Conthe bleaching process: Namely that bleaching is sequently, somewhere in the process must be inessentially delignification in the early stages and terposed a step for liberating individual fibers oxidizing or whitening in the later stages, and from the fiber bundles. that little orno fiber deterioration occurs in the earlier or delignificatiori stages. I have found that certain definite benefits are achieved by placing the step of mechanical refining in that sequence of bleaching operations near the end of thedelignification stage, yet preliminary to oxidation or whitening to a high degree. This drated pulp with 'hypochlorite solution to the mechanical refining may be done after the first hypoohlorite stage in processes where hypochlorite treatment follows treatments by chlorine, gaseous or liquid, and by alkaline solutions. However, I may, and preferably do, perform the refining step at a point following treatments with chlorine, gaseous or liquid, and with alkaline solutions, but preliminary to treatment with hypochlorite or like oxidizing material.

By breaking up the fiber bundles before oxidation is brought about, the process set forth in this invention achieves a desired evenness in bleaching and attains high degrees of whiteness with treatments less drastic than hitherto considered necessary. Economy of bleaching agent is a coincidental advantage. The novelty of this invention, however, lies in an improvement in the means of attaining these known benefits. In the delignification steps, particularly chlorination and the subsequent alkaline treatment, the interfibrous material remaining after digestion is softened to a considerable degree. Fiber liberation then can be accomplished without undue power consumption and without undue physical degradation of the fibers, such as often results from the drastic treatment necessary for refractory, highly lignified fiber bundles. As further advantages, this process is suitable for processing pulp which is digested more on the raw side than usual, and which consequently has high strength and yield characteristics.

An object of the present invention is to produce, in an economical manner, a final refined pu p:

and bleached pulp product having uniform whiteness. Another object of the invention is the provision of a refining and bleaching process wherein fiber degradation is minimized, so that the product thereof has improved properties as contrasted with prior art processes. Another object is to provide a process in which an econ omy of bleaching agent may be effected over those processes heretofore employed. A further object of the invention is to produce a refined and bleached pulp of higher yield, based on the equipment used; than those available in the earlier processes.

I have found that the above, and other, objects are effected by the carrying out of the following operations in the sequence named:

(1) Digesting the wood (or its equivalent) in a solution of the usual digestion chemicals-preferably terminating the digestion while the product is still on the raw side;

2) Washing thejigestio'n chemicals and spent cooking liquor from the resulting pulpy'preferably followed by the screening of the washed p p; 3) Treating the pulp with chlorine (either chlorine gas or chlorine water) in'anamount sufficient to' chlorinate the lignin content of the (4) Treating the pulp with mild caustic alkali solution under conditions and in an amountsufficient to'solubilize the chlorinated compounds content thereof;

(5) Washing soluble ingredients from the pulp; (6) As an optional step which may-be omitted, treating the" pulp with hypochlorite solution in an amount insufficient to cause appreciable whitening or oxidation i ('7) Refining and hydrating the pulp to a'degree just short ofthatdes'ired in the manufactureof the required paper product from the pulpy" (8) Bleaching the partially refined and hydesired standard of whiteness;

(9) Finally hydrating the bleached pulp, in a much milder manner than at stage ('7) above, to the exact degree of hydration appropriate to form the pulp into the desired paper product.

By this process it is possible to effect a better yield, of a stronger and otherwise superior pulp of an arbitrary standard of whiteness, with simultaneous improvement in economy of chemicals.

In accordance with this process, wood is prepared and digested in a manner common to any of the conventional chemical pulping processes. I need not, and preferably do not, carry the digestion to the point at which substantially all of the fiber bundles have been opened up, but rather may and preferably do-stop the digestion while the pulp is still slightly undercooked or "on the raw side. The digestion mixture, after discharge from the digester, and after or coincident with separation of the pulp from the liquids content of the mixture, may if desired be passed through the conventional equipment for the removal of knots, large portions of undercooked wood, dirt and the 'like. While this latter step usually is desirable it may not be necessary in all cases.

The pulp atthis stage is distinctly brown in color, and contains a relatively large content of lignin or lignous compounds. For the safe removal of lignin or lignous compounds I treat the pulp with gaseous chlorine or with chlorine water in an amount sufficient to react with (i. e.,

chlorinate) the lignin content of the crude pulp.

In carrying out the chlorination I prefer for economical reasons to react the chlorine with the pulp while the latter is in a relatively concentrated state (i. e.-, a high-density pulp), and

to use only enough chlorine to effect the afore-' said chlorination of the lignin compounds content of the pulp. I

The product of the chlorination step is adecidedly brown pulp-darker in color than was the pulp at the time of its discharge from the digester-due to the brown color of the chlorinated lignin compounds. These latter are not soluble in water as to be removable .from the pulp by washing, Consequently, I solubilize the same by a treatment of the pulp with a suitableamount of dilute caustic alkali (e. g-., caustic soda) .-solution, with agitation ofthe pulp and solution to effect satisfactorily thorough permeation of the pulp with the base. Thereafter, the so-treated pulp is thoroughly washed with water to free the pulp from soluble ingredients.

If desired, the pulp at this point may be 'sub-' 'jected to treatment with hypochlorite (e. g., calcium hypochlorite) bleaching powder solution 'or suspension, in an amount insufficient to cause appreciable oxidation or whitening. This treatment may be made at room temperature or at eIeVatedE temperature, and the pulp thereafter may be washed withwater, "It will be'recognized that little or no deterioration through oxi dation has" occurred uptothis point. However, this step may be,fand preferably'is, omitted at thispointein the sequence of operations.

Thereupon, the pulp is subjected to. refining and beating treatment to an extent just short of that degree, of refining and hydration desired for the manufacture o f-the eventual paper or "paper-product. Eorthis purpose the pulp may be refined and hydra;ted, ,for,example-, in a j ordan, whereby fiber bundles are opened up and reduced 8,186,084 to ultimate fibers and the cellulosic flbersare somewhat hydrated. v

This partially, or nearly completely, refined and hydrated pulp is then subjected to an oxidizing bleach by treatment thereof with a hypochlorite (e. g., calcium hypochlorite) or bleaching powder solution or suspension, at room temperature or at elevated temperature, and thereafter may be washed with water. Because of the homogeneity and openness of the pulp at this stage, the above oxidizing bleach step requires materially less of the chemical or chemicals than that amount heretofore considered necessary to attain comparable whiteness of the product.

The so treated and bleached pulp is finally given a very mild beating in a jordan or equivalent hydrating machine, whereby to bring the already substantially refined, and hydrated pulp to the exact degree of hydration indicated for best results in formation of the desired paper web, after which step the pulp may be used directly in a paper-web-forming operation.

I claim:

1. Process of preparing a bleached paper stuii', which comprises chlorinating the chlorinatable lignin compounds content of a chemically produced raw pulp by treatment of the latter with chlorine as such; solubilizing the chlorinated lignin compounds content by treating the pulp with mild caustic alkali solution andwashing the so-solubilized compounds from the pulp; beating and partially hydrating the pulp to a degree just short of that required for hydrating the pulp to a degree approximating that required for papermaking; subjecting the substantially refined and hydrated pulp to an oindizing bleach with an hypochlorite under conditions to bleach the pulp; and mechanically hydrating the bleached pulp, to the degree of hydration appropriate for papermaking, under mild hydration conditions.

2. Process of preparing a bleached paper stufi, which comprises chlorinating the chlorinatable lignin compounds content of a chemically produced raw pulp by treatment of the latter with chlorine as such; solubilizing the chlorinated lignin compounds content by treating the pulp with mild caustic alkali solution and washing the so-solubilized compounds from the pulp; treating the pulp with hypochlorite solution in amount insumcient to cause appreciable bleaching; beating and partially hydrating the pulp to a degree just short of that required for papermaking; subjecting the substantially refined and hydrated pulp to an oxidizing bleach with an hypochlorite under conditions to bleach the pulp;

and mechanically hydrating the bleached pulp, to the degree of hydration appropriate for papermaking, under mild hydration conditions.

.tent of the raw pulp by treatment of the latter with chlorine as such, (d) treating the product of step (c) with caustic soda solution in an amount suflicient to solubilize its content of chlorinated compounds, and washing the pulp free of the soluble components, (e) partially refining and hydrating the product of step (d) to a degree just short of the desired hydration for papermaking, (I) bleaching the substantially refined and hydrated pulp product of step (6) with hypochlorlte bleach to stahdardwhiteness. and (g) mildly mechanically hydrating the bleached product of step (I).

4. In the process of preparing a bleached pulp product suitable for direct use in paper-making, the sequence of operations which essentially consists of (a) digesting wood by a chemical pulping process, (b) separating liquids from the resulting raw pulp and screening the latter, (0) chlorinating the chlorinatable lignin compounds content of the raw pulp by treatment of the latter with chlorine as such, (d) treating the product of step (c) with caustic soda solution in an amount suflicient to solubilize its content of chlorinated compounds, and washing the pulp free of the soluble components, (e) treating the product of step (d) with an amount of hypochlorite insuflicient to produce appreciable oxidation or whitening, (1) partially refining and hydrating the product of step (ell to a degree just short of the desired hydration for paper-making, (g) bleaching the substantially refined and hydrated pulp product of step (f) with hypochlorite bleach to standard whiteness, and (h) mildly mechanically hydrating the bleached product of step (g).

. HAROLD R.

CERTIFICATE OF CORRECTION. Patent No. 2,1 6,o u. January 9, 191m.

HAROLD R. MURDOCK. 7

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, first column, lines 55 and 51+,- claim 1, strike out the words "hydrating the pulp to a degree approximating that required for and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office Signed and sealed this 15th day of February, A. D. l9h0 I I Henry Van Arsdale, (Seal) Acting Commissioner of Patents. 

